If you are not using riseup, you need to replace 172.27.100.1 and enter the virtual LAN IP address of your VPN providers DNS server. You might be able to obtain it from your VPN provider. You can also try to infer it after successfully connecting to the VPN from running "sudo route". The first destination default gateway should function as DNS server also.

Save.

If you want to be sure, that /etc/resolv.conf does not get overwritten by other packages. (Such as DHCP or resolvconf.)